1. Field of the Invention
The present invention relates to a wireless communication system, and particularly, to a system and a method for maintaining cache consistency in a wireless communication system.
2. Description of the Background Art
Due to development of wireless communication technology and popularization of portable communication devices, many wireless communication environments are introduced.
Mobile transactions in a wireless communication environment, typically, are comprised of multiple sub-operations. The successful execution of a mobile transaction is therefore dependent on successful execution of all its sub-operations.
Execution of a transaction in a wireless communication environment is limited by mobility of the mobile communication terminal, disconnection frequency, limited bandwidth, battery capacity, etc. Therefore, in order to efficiently use the limited bandwidth and to improve response time, a caching method is used.
Referring to FIG. 1, a typical wireless communication system comprises a mobile communication terminal 10 for caching data which is accessed frequently; and a communication server 20 for wirelessly communicating with the mobile communication terminal 10.
In case that the data is updated in the communication server 20, the communication server 20 broadcasts an invalidation report (IR) to update the cached data to maintain the consistency of the cached data. Since the wireless communication network has limited bandwidth, it is desirable that the communication server 20 notifies the mobile communication terminal 10 of the data update as little as possible. Therefore, the communication server 20 does not transmit the IR to the mobile communication terminal 10 whenever the data is updated, but it broadcasts the IR in certain intervals.
However, the mobile communication terminal 10 and the communication server 20 are frequently disconnected due to the mobility of the mobile communication terminal and the limited battery capacity. Therefore, since the mobile communication terminal 10 cannot receive the IR broadcasted from the communication server 20 due to the frequent disconnections, the cache consistency is not maintained. Thus, a system and method for maintaining cache consistency between the mobile communication terminal 10 and the communication server 20 is needed.
FIG. 2 shows a method for maintaining cache consistency in a general wireless communication system. FIG. 3 shows a method when the IR is broadcast regularly from the communication server to the mobile communication terminal for maintaining the cache consistency.
Generally, the communication server 20 broadcasts data which can be used in mobile transaction process by the mobile communication terminal 10. When the entire data reaches the mobile communication terminal 10, the mobile communication terminal 10 caches the data which is accessed frequently in the entire data, in the cache (i.e., transaction cache).
The communication server 20 broadcasts information (i.e., invalidation report and entire data) during several broadcasting periods (w) at each broadcast period (L). The invalidation report and the entire data may be broadcast simultaneously, or the entire data may be broadcast right after the invalidation report, in case that one forwarding channel is used. Whether or not the IR and the entire data are broadcast simultaneously is not important.
The IR includes broadcast time stamp (Ti) representing the time of present broadcast period and update list (j, tj). j represents updated data item (identifier of the data) and tj is updated time stamp representing the last updating time of the data (j). Several broadcast periods w (w>1) can be set to be maximum interval through which the mobile communication terminal 10 can endure the disconnection.
The mobile communication terminal 10 receives the entire cache data and update history that occurred during the previous several broadcast periods w by way of periodical broadcasts from the server 20 (S11). The mobile communication terminal 10 then determines whether or not there was a disconnection within a range which the mobile communication terminal 10 could not endure (S13). The mobile communication terminal 10 has a last receive time stamp tslb representing the latest reception time of the IR, and compares the last receive time stamp to the broadcast time stamp Ti of the received IR.
If the time difference between the broadcast time stamp Ti and the last receive time stamp is not larger than above w, the mobile communication terminal 10 decides that there was not a disconnection that the mobile 10 communication terminal 10 could not endure, and updates the corresponding data of the cache by the received IR to maintain the cache consistency (S15). Additionally, the mobile communication terminal 10 assigns the last receive time stamp as the broadcast time stamp Ti of the received IR.
However, if the time difference between the broadcast time stamp Ti and the last receive time stamp is larger than w, the mobile communication terminal 10 decides that there was a disconnection that the mobile communication terminal 10 could not endure, and drops all data cached on the cache (S17). In addition, the mobile communication terminal 10 requests the communication server 20 to provide the entire cache data, and updates the entire cache data (S19).
As described above, according to the general method for maintaining the cache consistency, the mobile communication terminal is able to deal with the disconnection for a predetermined period (w) by broadcast periodically the update history during the previous several broadcast periods. According to the above method, the information (i.e., IR and entire data) occurred during the several broadcast periods is broadcast periodically, and thereby, the narrow bandwidth is not used efficiently. Also, the IR includes the update time stamp and the identifier of the updated data, and thereby, the information bits included in the IR are increased.